Serum and red blood cell folate testing for folate deficiency: new features?

INTRODUCTION Folate deficiency is assessed by serum and red blood cell folate measurements. Nevertheless, no consensus for the lower limit of serum folate reference values exists. We investigated the appropriate use of RBC folate to detect folate deficiency and the relationship between serum and RBC folate and with other parameters such as vitamin B12 and homocysteine in order to propose serum folate cut-off values. METHODS Retrospectively, 63,113 and 20,459 results of serum and RBC folate were collected. If present, the results of red cell indices, vitamin B12 and homocysteine were also collected. RESULTS A significantly positive correlation between serum and RBC folate was demonstrated. A significant effect of serum folate levels under 6 μg/L (or 14 nm) was observed on RBC indices. A relation was found between vitamin B12 and folate, for serum and RBC. A significant rise in homocysteine concentrations was observed for serum folate levels under 8 μg/L (or 18 nm). CONCLUSION To observe haematological abnormalities, folate deficiency should be profound. Serum folate levels under 8 μg/L (or 18 nm) should be considered as a decision limit for folate depletion because a positive effect on homocysteine was observed. Fasting serum folate concentration should be preferred for assessing folate status. Our results suggest that the need for RBC folate testing is less meaningful.